Methods and apparatus for measuring the rate of penetration in well drilling

ABSTRACT

New and improved methods and apparatus exemplifying the present invention are disclosed herein for measuring at the surface the changes in total elongation of a drill string due to variations in the tensional forces acting thereon during a typical well drilling operation, measuring the changes in the total length of the drill string as it is moved into and out of the borehole at the surface, and uniquely combining these measurements for producing an output signal which is representative of the changes in the borehole depth as well as converting the combined measurements for producing another output signal which is representative of the actual rate of penetration of a drill bit coupled to the drill string.

United States Patent [191 Guignard METHODS AND APPARATUS FOR MEASURING THE RATE OF PENETRATION IN WELL DRILLING [75] Inventor: Jean-Hubert Guignard, Sainte Mesne, France [73] Assignee: Schlumberger Technology Corporation, New York, NY.

[221 Filed: Dec. 13, 1971 [21] Appl. No.: 207,326

[30] Foreign Application Priority Data Dec. 11, 1973 2,330,753 9/1943 Sikes, Jr 73/151 UX 2,166,212 7/1939 Hayward 73/151 X FOREIGN PATENTS OR APPLICATIONS 2,03 8,700 8/1971 France Primary Examiner-Jerry W. Myracle Attorney-Ernest R. Archambeau, Jr. et a1.

[57] 7 ABSTRACT New and improved methods and apparatus exemplifying the present invention are disclosed herein for measuring at the surface the changes in total elongation of a drill string due to variations in the tensional forces acting thereon during a typical well drilling operation, measuring the changes in the total length of the drill string as it is moved into and out of the borehole at the surface, and uniquely combining these measurements for producing an output signal which is representative of the changes in the borehole depth as well as converting the combined measurements for producing another output signal which is representative of the actual rate of penetration of a drill bit coupled to the drill string.

22 Claims, 3 Drawing Figures PROCESSING AND RECORDING PATENTH] 0E6 1 1 I973 SHEET 10F 2 PROCESS ING AND RE CORD/N6 E O R L ewmw -wwm 8 S R W NN Em M R B C X\\\ I D N {A m 7/ UN B V s w 7 D N L 5 Jean Hubert Guignard IN VE N TOR flaw ATTORNEY METHODS AND APPARATUS FOR MEASURING THERATE OF PENETRATION IN WELL DRILLING It will be appreciated by thoseskilled in the art that it is of considerable importance to know the actual rate at which the drill bit is penetrating the earth formations during the course of a typical well drilling operation. Forexample, if this so-called rate of penetration can be measured with reasonable accuracy, the influence of controlled variables (such as the rotational speed and the weight imposed on the drill bit) on the rate of penetration can be determined thereby enabling optimum drilling rates to be maintained. Various proposals have,

of course, been advanced heretofore for hopefully determining the actual rate of penetration by making one or more surface measurements. For example, one common technique is to arrange a transducer which is responsive to the longitudinal movements of the drill string in relation to the rig floor for producing electrical signals which are representative of the displacement rate of the drill string at the surface.

It will be recognized, however, that the displacement rate of the upper portion of the drill string at the surdrill string during the course of a drilling operation.

By way of explanation, the usual practice in drilling a well is to arrange one or more drill collars in the drill string just above the drill bit for imposing a downward force on the drill bit which is no more than the combined weight of the drill collars and to employ a series of tandemly-coupled joints of drill pipe for the remainder of the drill string extending to the surface. To prevent buckling of the drill pipe in the string, an upward force or so-called hook load is typically maintained on the surface end of the drill string to assure that the drill pipe is always in tension. By observing a so-called weight indicator operatively coupled to the surface end of the drill string, this upward force is maintained within a predetermined rangewhich is somewhat less than the total weight of the drill string but at least slightly greater than the total weight of the drill pipe in the drill string so that at least some if not a major portion of the total weight of the drill collars will always be imposed on the drill bit during the drilling operation to achieve a desired weight on bit.

Atypical drilling procedure is to set the drill bit on the bottom of the borehole after an additional joint of drillpipe has been coupled to the upper end of the drill string, impose a selected minimum upward force on the drill string which iswithin the aforementioned range of forces, and then resume rotation of the drillstring to continue the drilling operation without concurrently lowering the drill string. Thus, as the drill bit successively excavates the borehole, the drill string will be progressively elongated which concurrently reduces the downward force acting on the drill bit (weight on bit) and increases the upward force acting on the surfaceend of the drill string (hook load). Once the surface measurements indicate that this upward force has reached a selected maximum corresponding to a desired minimum downwardly-acting force on the drill bit, the drill string is lowered to again reduce the upward force on the drill string to the aforementioned selected minimum value. It will be appreciated, therefore, that the forces acting on the surface end of the pipe will be successively increased from this selected minimum force to the selected maximum force and then reduced as the cycle is repeated.

As a result, it will be recognized that the drill string is constantly subjected to varying tension forces which cause the drill string to be correspondingly elongated. The degree of elongation is, however, not at all inconsequential. For example, with a drill string composed substantially of 10,000-feet of a typical 4 i-inch diameter drill pipe, tension forces in the order of 140,000 lbs. are not at all uncommon. Moreover, if this force is varied only 1,000-lbs. for a period of only l-minute, it can be shown that the difference between the actual rate of penetration of the drill bit and the rate of displacement in the drill string as measured at the surface will be in the order of S-feet/hour. This would, of course, represent a serious error since drilling rates are commonly as low as 2 to 3-feet/hour; and an error of this magnitude would obviously preclude accurate control of the drilling operation.

Accordingly, it is an object of the present invention to provide new and improved methods for accurately determining from surface measurements the rate of penetration of a drill bit while drilling a borehole.

Another object of the invention is to provide new and improved apparatus for measuring the instantaneous drilling rate of a borehole after eliminating movements of the drill string which do not correspond to the actual drilling through the "formations.

These and other objects of the present invention are attained by providing methods and apparatus for measuring the changes in the elongation of the drill string in the borehole; measuring the incremental lengths of the drill string being moved into the borehole from the surface during a drilling operation; and, after combining these two measurements, converting them in such a manner as to derive information at the surface which is representative of the actual rate of penetration of the drill bit.

The novel features of the present invention are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may be best understood by way of the following description of exemplary apparatus and methods employing the principles of the invention as illustrated in the accompanying drawings, in which:

FIG. 1 shows a typical drilling rig including new and improved apparatus according to the invention;

FIG. 2 is a block diagram of a preferred embodiment of new and improved apparatus arranged in accordance with the present invention and especially adapted for practicing the methods of the invention; and

FlG. 3 depicts a portion of the unique circuit shown in FIG. 2.

Referring to FIG. 1, a typical rotary drilling rig 10 is shown with a crown block 11 over which runs a cable 13 driven by a draw works 18 adapted for operatively controlling the upward and downward movements of a traveling block 12. The traveling block 12 carries a hook 14 from which is suspended a rotary swivel 15 supporting a drill string 16 comprised of a number of tandemly-coupled joints of drill pipe and drill collars. As is typical, the drill string 16 carries a drill bit 17 at its lower end and is dependently supported at its upper end from a kelly joint 20 which is rotatively driven by a rotary table 21 on the rig floor. Drilling mud is delivered to the drill string 16 in the usual fashion by a mud hose 23 coupled in the usual fashion to the swivel 15.

To practice the new and improved methods of the present invention with the drilling rig 10, it will be appreciated that downward movements of the drill bit 17 resulting in further excavation of the borehole are substantially achieved by moving the surface end of the drill string 16 downwardly as by lowering the traveling block 12. As previously described, however, the drill string 16 is constantly subjected to significant tensional forces which continuously vary during the course of a drilling operation. Since these tensional forces are typically of substantial magnitude and the drill string 16 usually has a considerable length, it has been found that the accurate determination of the actual rate of penetration of the drill bit 17 requires that the resulting variations in the overall length of the drill string caused by its elongation and contraction under these changing tensional forces be taken into account.

Measurements of the vertical displacement of the drill string 16 are, of course, best measured at the surface. Various techniques for making these measurements are known to those skilled in the art. To determine the changes in the overall length of the drill string 16 caused by variations in the tensional forces acting on the drill string, these varying forces can be measured at any place in the drill string such as, for example, by conventional strain gages. It should also be noted that since the total weight of the drill string 16 is equal to the summation of the aforementioned hook load (typically measured at the surface) and the aforementioned weight on bit (typically measured downhole), a change in either of these two forces will be representative of a change in the tensional forces acting on the drill string.

In the preferred manner of practicing the present invention during the course of a drilling operation, a series of measurements are continuously obtained for successively producing a series of first signals which are representative of the incremental vertical displacements of the surface end of the drill string 16. Simultaneously therewith, another series of measurements are continuously obtained which are representative of incremental changes in tensional forces acting on the drill string 16. These latter measurements are successively converted by a predetermined factor which is functionally related to the total length of the drill string 16 and its coefficient of elongation for successively producing a series of second signals which are representative of the incremental elongational changes in the overall length of the drill string. Thereafter, the first and second signals are combined to obtain a series of successive third signals which are representative of the net downward incremental distances traveled by the drill bit 17 during the measuring period. By determining the rate of these successive combined signals, indications of the actual rates of penetration of the drill bit 17 are continuously provided at the surface. Moreover, by continuously totaling these combined third signals, indications of the total distance traveled by the drill bit 17 into the formations during the measured period are also presented at the surface.

In keeping with the objects of the present invention, apparatus is provided for producing a series of first signals which are representative of the direction as well as incremental vertical displacements of the drill string 16 during the course of a drilling operation with the drilling rig 10. Accordingly, in the preferred embodiment of the present invention, displacement-responsive transducer means 24 are placed on top of the drilling rig 10 and operatively associated with one of the pulleys in the crown block 11 to respond to the rotation of the pulley for successively producing a series of first electrical signals which are representative of the direction as well as the incremental distances traveled by the surface end of the drill string 16. As will be subsequently explained, the output signals from the displacement transducer 24 are transmitted by means such as a suitable electrical cable 25 to a new and improved signal processing and recording unit 26. Furthermore, in the preferred embodiment of the present invention, elongation-responsive means including a forceresponsive transducer 27 coupled between the swivel 15 and the kelly 20 are operatively arranged for successively producing a series of second electrical signals which are representative of incremental changes in the length of the drill string 16.

Turning now to FIG. 2 showing the preferred embodiment of the present invention, the displacmentresponsive transducing means 24 are illustrated as including a lamp 31 directed toward one side of a rotatable disk 32 having a series of either peripheral notches or alternately transparent and opaque zones spaced around the rim of the disk. When the disk 32 is rotatively driven by the crown block pulley 11, the light from the lamp 31 periodically strikes a photodiode 33 arranged on the other side of the disk for producing, at an output X, a train of successive pulses at a frequency or pulse rate which is proportional to the rotational speed of the disk. In a similar fashion, a second photodiode 34 is arranged on the other side of the disk 32 for producing, at an output Y, a second train of pulses having the same frequency or pulse rate as the first train but which are phase-shifted in relation thereto. it will be appreciated, therefore, that the second pulse train either leads or lags the first pulse train depending upon the rotational direction of the disk 32.

The photodiodes 33 and 34 are linked via the cable 25 to a typical directional logic circuit 35 which is included in the processing and recording unit 26. This circuit 25, which may include a shaping input circuit, is well known by those skilled in the art and is cooperatively arranged for generating a series of positive pulses, +V,, when the disk 32 rotates in one selected direction and a series of negative pulses, V,,, when the disk 32 rotates in the other direction. Thus, as a matter of choice, the circuit 35 is arranged so that a downward movement of the drill string 16 will produce a corresponding series of positive pulses, +V,,, and an upward movement of the string will conversely produce a corresponding series of negative pulses, V,, with each pulse being representative of a selected increment of length. It will be seen, therefore, that the signals, +V, and V,, are alternative output signals from the logic circuit 35, with the number of these pulses being representative of the total distance traveled by the upper portion of the drill string 16 and their frequency or (pulse rate being respectively proportional to the rate of travel or vertical displacement of the drill string at the surface. The polarity of these pulses will, of course, indicate the direction of travel.

lnthe preferred embodiment of the present invention, the transducer 27 coupled to the upper part of the drill string 16 is comprised of a strain gage bridge 38 whose opposite ends are connected to the input of an amplifier, 36. The bridge 38 is thermally compensated to minimize its residual drift thereby making it possible to accurately measure small variations in the tension forces acting on the drill string 16. In the preferred embodiment of the transducer 27,. the bridge 38 is arranged so that the maximum drift will be in the order of only lO-kilograms/minute. With a minimal drift of this order, weight variations of only about 33-kilograms over a period of l-minute can be sensed by the bridge 38 to provide an accuracy of about 0.1-meters/hour in measuring the rate of penetration of the drill bit 17.

The output signal, W, of the amplifier 36 is coupled by the cable 28 to a weighting circuit 37 comprised, for example, of a variable resistor 40 for multiplying the output signal, W, of the amplifier 36 by a selected coefficient, K, which is a function of the elasticity of the drill pipe in the drill string 16.

By virtue of Hookes Law, it will be recognized that the coefficient, K, is determined by dividing the total length of the drill pipe in the drill string by the product of the transverse cross-sectional metal area and the modulus of elasticity of this drill pipe. Thus, by multiplying the force, W, sensed by the transducer 27 by the coefficient, K, the variations in the resulting signal, KW, will be representative of the changes in the elongation of the drill string 16 which are produced by variations in the tension force, W. It will, of course, be realized that should different types of drill pipe be coupled into the drill string 16, the coefficient, K, will be determined by simply adding the individual coefficients calculated for each type of drill pipe included in the string 16 so as to provide an overall or a composite value for the coefficient, K, for producing the signal, KW. It

K, when the movable contact is at the ungrounded end of the resistance element. Thus, for lesser values of K, the movable contact will be selectively set at an appropriate intermediate portion on the resistance element of the potentiometer 40. Although the potentiometer 40 theoretically requires repositioning each time an additional joint of drill pipe is coupled into the string 16, it has been found that as a practical matter sufficientlyaccurate measurements are obtained in the practice of the present invention by readjusting the potentiometer at only infrequent intervals. For example, when the drill bit 17 is drilling in hard formations, adjustments of the potentiometer 40 may be made only once every 12 to 24-hours since drilling speeds are typically so low in such formations that the overall length of the drill string 16 is not significantly increased in such a time interval.

The output of the weighting circuit 37 is coupledto the input of a quantized differentiating circuit 41 which, as will subsequently be described in more detail in relation to FIG. 3, is operatively arranged for producing a series of output signals which are proportional to incremental changes in the overall length of the drill string 16 caused by the elongation and contraction of the drill string. In the preferred embodiment of the present invention, this is accomplished by comparing the signals, KW, with two comparison signals, +8 and S, of opposite polarity and respectively having a selected magnitude of an equal value. The magnitude of these comparison signals is selected so that the differentiating circuit 41 will produce "alternative series of output pulses, +V or V,,, having a pulse rate or frequency which is representative of the rate of change in elongation of the drill string, with each pulse being representative of a selected incremental unit of change in length of the drill string 16. In this manner, the positive output pulses, +V from the differentiating circuit 41 will be successively produced in response to incremental elongations of the drill string 16 and the negative output pulses, V,,, will be successively produced in response to incremental contractions of the drill string.

Accordingly, as described to this point, it will be appreciated that the new and improved apparatus of the present invention includes displacement-responsive means (as exemplified by the transducer 24 and the logic circuit 35) operatively arranged for producing a first series of output pulses, either +V, or -V,, which are respectively related to a selected incremental length of the drill string 16 which has been moved either downwardly or upwardly in relation to the rig floor. The frequency or pulse rate of the first series of pulses, +V, and V,,, will be proportional to the rate of travel of the drill string 16 and the polarity of the pulses will indicate the direction of the movement. Moreover, the elongation-responsive means of the present invention (as exemplified by the circuit 42) are adapted for producing a second series of output pulses, either or V,,, which are respectively related to incremental elongational changes in the overall length of the drill string 16. The frequency or pulse rate of these second pulses will be proportional to the rate of the change of the overall length of the drill string 16; and their polarity will indicate whether the overall length of the string has been increased or decreased.

It will be recognized that the drill bit 17 will be penetrating an earth formation only upon downward movements of the drill bit whether these movements are caused by further elongation of the drill string 26, an actual downward movement or displacement of the drill string at the surface, or a combination of these two movements. Moreover, it should be appreciated that the actual displacement movements of the drill string 16 are independent of the elongational changes of the drill string. For example, as previously explained, the usual drilling practice is to periodically retain the upper end of the drill string 16 at a selected position in relation to the rig floor and allow it to further elongate as the drill bit 17 continues to deepen the borehole. Conversely, as the drill string 16 is periodically lowered to increase the weight imposed on the drill bit 17, the drill string will be contracted as the tensional forces acting thereon are correspondingly reduced. In this latter situation, it will be recognized that the downward travel of the drill string 16 will be partially offset by the attendant relaxation of the drill string so that during this time the actual rate of penetration of the drill bit 17 will be correspondingly less than the rate of downward travel of the surface end of the drill string.

In keeping with the objects of the present invention, therefore, the new and improved apparatus disclosed in FIG. 2 is further arranged for combining the aforementioned signals, V, and V,,, and then converting the combined signals for providing accurate indications at the surface which are representative of the actual rate of penetration of the drill bit 17. To accomplish this, the positive signal outputs of the displacementresponsive meansand' the elongation-Tesponsive means are respectively coupled to the inputs of an OR gate '48 for producing a series of positive output pulses, +V in response to the generation of either positive displacement pulses, +V,,, or positive elongation pulses, +V or both. Similarly, the negative signal outputs of the displacement-responsive means and the elongationresponsive means are respectively coupled to the inputs of an OR gate 49 for producing a corresponding series of negative output pulses, V in response to the generation of either or both of the negative pulses, V and -v,.

To prevent the displacement-responsive pulses, +V, or V,,, from being masked by the elongationresponsive pulses, +V or V,,, the circuits supplying these pulses to the gates 48 and 49 are respectively designed to generate very short pulses thereby reducing the probability that two of these pulses may appear simultaneously at the inputs of the gates 48 and 49. Thus, in the preferred embodiment of the present invention, shaping circuits such as monostable multivibrators or one shots 43 and 44 are added to the outputs of the logic circuit 35 to minimize the duration of the pulses, +V, and V,. It will also be noted that instead of being alternative signals as are the signals, +V, and V,, or +V and V,,, the combined signals, +V and V,,, may simultaneously appear at the outputs of the OR gates 48 and 49.

As previously mentioned, the actual rate of penetration of the drill bit 17 is decreased by contractions of the drill string 16 in response to decreases of the tensional forces acting thereon as well as by any upward movements of the drill string. Conversely, the actual rate of penetration of the drill bit 17 is increased both by downward movements as well as by further elongations of the drill string 16. Accordingly, the output signals, +V and V,,, from the OR gates 48 and 49 are respectively supplied to the addition and subtraction inputs of a typical reversible counter 46 such as shown in French Pat. No. 1,541,771. For reasons that will subsequently be explained, the outputs of the OR gates 48 and 49 are respectively coupled to the counter 46 by way of AND gates 47 and 45 which, as far as is necessary for understanding the present invention to this point, are both always operatively enabled so that as long as the drilling operation is actually progressing the V signal will be applied to the counter for algebraic summation.

As illustrated in FIG. 2, the +V signals from the AND gate 47 are simultaneously supplied to one input of an AND gate 50 having its output coupled to the addition input of the counter 46 as well as to one input of an AND gate 51. The output stages of the counter 46 are connected to an OR gate 52 whose output is connected, on the one hand, to the other input of the AND gate 50, on the other hand, to an inverter 53. The output of the inverter 53 is connected to the second input of the AND gate 51. When the counter 46 is in a zero" state, the output signal of the OR gate 52 is a zero signal which inhibits the AND gate 50 and enables the AND gate 51. For all other states of the counter 46, the AND gate 51 is inhibited, the pulses +V then being applied to the input of the counter for summation.

It will be seen, therefore, that the circuit 54 including the counter 46, the operatively-arranged gates 50-52, and the inverter 53 is responsive to the displacement signals, +V and V,,, for providing a third signal, V,,, only when there is a downward advancement of the drill bit 17 during a drilling operation as a result of either downward displacement of the drill string 16 at the surface or an elongation or increase in the overall length of the drill string. On the other hand, the circuit 54 is cooperatively arranged so that an upward displacement of the drillstring 16 will discontinue the production of further output pulses, V,,, until the drill bit 17 is again at its previous lowermost depth to continue further excavation of the borehole.

By way of explanation, it,will be appreciated that each time there is either an incremental upward movement of the drill string 16 at the surface (V,) or an in cremental shortening of the drill string (-V the OR gate 49 will always supply a negative pulse, V directly to the subtracting input of the counter 46. Similarly, each time the OR gate 48 supplies a positive pulse, +V representative of either an incremental downward movement or an incremental lengthening of the drill string 16, the OR gate52' is selectively responsive to the present state of the counter 46 for either di recting the positive pulse to the adding input of the counter or for producing a pulse, V at the output of the gate 51. For example, assume that the drill bit 17 has been steadily moving downwardly so that the output of the OR gate 52 is a zero signal signifying that the counter 46 is in a zero state. The OR gate 52 will, therefore, inhibit the AND gate 50 and enable the AND gate 51 so that each positive pulse, +V at the input of the circuit 54 will simultaneously produce an output pulse, V,,, at the output of the circuit 54. Under this situation, a continuation of positive input pulses, +V will produce a corresponding series of output pulses, V having the same pulse rate.

A single negative input pulse, -V,,, signifying either a shortening of the drill string 16 or an upward movement of the drill string at the surface will, however, be applied to the subtracting input of the counter 46 to produce an output signal from the OR gate 52 which is representative of the counter being in a negative or non-zero state. The OR gate 52 then inhibits the AND gate 51 to discontinue further production of the output signals, V until the counter 46 is again in a zero state and concurrently enables the AND gate 50 to direct subsequent positive input pulses, +V to the adding input of the counter.

It will be appreciated, therefore, that the continuation of negative input pulses, V,,, will maintain the condition of the circuit 54 and no output signals, V,,, will be produced which is,'of course, representative of no advancement of the drill bit 17 and a zero rate of penetration. On the other hand, assume that this situation was in response to either a momentary elevation of the drill string 16 at the surface or a brief reduction in the tension load sensed by the strain gage 38 causing the drill string to shorten, once either a downward movement or a lengthening of the drill string occurs to produce a positive input pulse, +V this positive pulse will be directed to the counter 46 to place it into a lessnegative state. Thus, once the number of positive pulses, -|-V,,, which are directed by the AND gate 50 to the counter 46 equals the number of negative pulses, V previously stored in the counter, the counter is again placed in a zero" state and the AND gates 50 and 51 are again respectively inhibited and enabled by the zero output signal of the OR gate 52. lt should be noted that the delay of this circuitry will prevent the final positive pulse, +V re-enabling the AND gate 51 from producing an output pulse, V,,. Once the counter 46 is restored to its zero state, the output pulses, V,,, will again be produced by the circuit 54 in response to the subsequent input pulses, +V

It should be noted that the counter 46 needs only to have a sufficient number of stages for storing a displacement equal to the vertical distance over which the traveling block 12 is capable of moving in relation to the floor of the rig since it is not necessary to totalize movments of the drill string 16 over any greater distance above the bottom of the borehole. A manual reset 55 is provided for resetting the counter 46 when drilling is to be resumed such as, for example, after a trip for changing the drill bit 17 or when the system 26 is to be energized.

Inasmuch as the frequency or pulse rate of the pulses, V,,, is proportional to the actual rate of penetration, circuit means 56 are provided for converting the frequency of these pulsesto an indication or record of the rate of penetration of the drill bit 17 during the course of a drilling operation. In the preferred manner of accomplishing this, the circuit means 56 include a frequency'to-voltage converter which is comprised of a monostable or one-shot circuit 57 followed by a lowpass filter 60 designed to cover the anticipated frequency or pulse rate output range of the pulses, V,,. It

will, of course, be appreciated that the low-pass filter 60 will cooperate to provide an output signal which is the average of the instantaneous drilling speed. Thus, for providing a record of the rate of penetration of the drill bit 17 as a function of time, the output of the frequency-to-voltage converter 56 is coupled to a time recorder 64. To provide a record of total depth as a function of time, the output pulses, V,,, are also coupled to the time recorder 64 by means such as a pulse divider 61 for printing a mark on the recording medium each time the drill bit17 has drilled an incremental depth corresponding to the predetermined distance assigned to each pulse.

It will also be recognized that the summation of the number of output pulses, V,,, is also representative of the total depth of the borehole as drilled at that time. Accordingly, to provide visual indications or a continuous record of the rate of penetration versus depth during the course of a drilling operation with the drill rig 10, the output pulses, V,,, are also employed for operating a stepping motor 62 which drives a recorder 63 to which the output of the converter 56 is coupled. If a rate of penetration indicator, as at 65, is desired, it can also be coupled to the output of the converter 56 for providing an instantaneousindication at some convenient location on the surface of the present application rate of the drill bit 17.

It will be recognized that when another joint of pipe is added to the drill string 16, the kelly 20 is temporarily disconnected and the drill string is suspended by slips placed in the rotary table 21. Various movements of the traveling block 12 are, of course, then employed in adding another joint of pipe to the drill string 16. Accordingly, to temporarily block the counter 46, a comparator 66 is coupled to the inputs of the AND gates 45 and 47. The inputs of the comparator 66 are respectively connected to a reference voltage source '67 and to the output of the amplifier 36. Thus, when the weight sensed by the elongation-responsive means 27 is less than a predetermined value, the comparator 66 then functions to inhibit the AND gates 45 and 47 for temporarily blocking the counter 46. The reference voltage is preferably chosen so that this blocking occurs at the moment when the tension measurement of the bridge 38 indicates that the drill string 16 has been placed on the slips. When the drill string 16 is again suspended from the traveling block 12, the AND gates 45 and 47 are re-enabled and the counter 46 then resumes its operation.

Referring to FIG. 3, a preferred embodiment of the quantized differentiating circuit 41 is shown in greater detail. The output signal, KW, of the weighting circuit 37 is applied to one input of a differential amplifier 70 having its output connected to one of the inputs of each of two comparators 71 and 72. As will subsequently be explained, equal voltages, +8 and S, of opposite polarity are each coupled to the other of the inputs of the two comparators 71 and 72 and the outputs of the comparators are respectively connected to the set and reset terminals of a bistable multivibrator 73 consist ing, for example, of two operatively-interconnected NAND gates 74 and 75. The outputs of the comparators 71 and 72 are also coupled to a NAND gate 76 which is connected to the control terminal of a multivibrator 77. The multivibrator 77 is connected to one of the inputs of each of the two AND gates 80 and 81 having their other inputs respectively coupled to the forward and reverse outputs of the bistable multivibrator 73. The outputs of the AND gates 80 and 81 which supply the output pulses, +V and V,,, of the differentiator 41, are respectively coupled to the adding and subtracting terminals of a reversible counter 82 whose output states are in turn connected to a binary-to-analog converter 83 which operatively supplies a control or feedback signal to the second input of the differential amplifier 70.

In operation, an increase in the input signal, KW, greater than the +S signal is detected by the comparator 71 which is operative for triggering the multivibrator 77 as well as setting the bistable multivibrator 73 for enabling the AND gate 80 and inhibiting the AND gate 81. A series of output pulses, +V,,, are generated by the multivibrator 77 and supplied to the addition input of the counter 82, with each pulse being representative of an incremental increase in length of the drill string 16 as established by the conversion factor of the D/A converter 83. As the contents of the counter 82 increase and are transformed into corresponding analog signals by the D/A converter 83, the proportionally-decreasing output signal of the differential amplifier finally be;

comes less than the +S signal, and the comparator 71 is then operative for inhibiting the multivibrator 77 to discontinue further production of the output pulses, +V,,. The operation of the circuit 41 is similar for a decrease in the amplitude of the input signals, KW, except that V,, output pulses will appear at the subtraction input of the counter 82.

Accordingly, it will be appreciated that the present invention has provided new and improved methods and apparatus for providing surface measurements of the actual rate of penetration of a drilling bit during the course of a drilling operation. As previously described, these new and improved methods and apparatus are uniquely arranged for accurately determining the distance traveled downwardly by the drill string and the increases in its overall length. By combining and then converting the combined measurements, instantaneous indications are provided at the surface which are representative of the true rate of penetration of the drill bit.

While only a particular embodiment of the present invention and one mode of practicing the invention have been shown and described, it is apparent that changes and modifications may be made without departing from this invention in its broader aspects; and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What is claimed is:

1. A method for determining the rate of advancement into earth formations by a drill bit suspended from a drill string in a borehole penetrating said earth formations and comprising the steps of: successively measuring the downward incremental displacments of the surface end of said drill string being moved into said borehole while said drill bit is drilling into said earth formations for producing a series of first electrical pulses respectively corresponding to an incremental increase to the length of said drill string in said borehole; successively measuring the incremental changes in tensional forces acting on said drill string while said drill bit is drilling into said earth formations and converting said force changes to corresponding increments of length of alternatively producing either a series of second electrical pulses respectively corresponding to an incremental elongation of said drill string or a series of third electrical pulses respectively corresponding to an incremental contraction of said drill string; comparing the total of said first and second electrical pulses occurring during a time interval after the occurrence of one of said third electrical pulses with the total of said third electrical pulses occurring during said time interval; once the total of said first and second electrical pulses counted during said time interval equal the total of said third electrical pulses counted during said time interval, successively processing said first and second electrical pulses for producing a series of electrical output pulses respectively corresponding to a net incremental increase in the overall length of said drill string until the occurrence of another one of said third electrical pulses; and successively converting said output pulses to a signal representative of the rate of downward advancement of said drill bit into said earth formations during a selected time interval.

2. The method of claim 1 further including the step of: totalizing said output pulses for providing an indication of the actual depth of said drill bit in said borehole.

3. Apparatus adapted for determining the rate of penetration into earth formations by a drill bit suspended from'a drill string in a borehole penetrating such formations and comprising: displacementresponsive means adapted for producing a series of first electrical pulses respectively corresponding to a downward movement of an incremental length of a drill string into a borehole during a drilling operation; elongatiomresponsive means adapted for alternatively producing either a series of sound electrical pulses respectively corresponding to an incremental elongation of the overall length of a drill string in a borehole during a drilling operation or a series of third electrical pulses respectively corresponding to an incremental contraction of the overall length of a drill string in a borehole during a drilling operation; processing means operatively coupled to said displacement-responsive means and to said elongation-responsive means and adapted to respond to said electrical pulses produced thereby for providing a series of electrical output pulses respectively corresponding to a net incremental increase in the overall length of a drill string in a borehole during a drilling operation so long as the total number of said first and second electrical pulses produced during a selected time interval exceeds the total number of said third electrical pulses produced during said selected time interval; and converting means operatively coupled to said processing means and adapted to respond to said output pulses for providing an output signal representative of the rate of penetration into earth formations by a drill bit while drilling a borehole during said selected time interval.

4. The apparatus of claim 3 wherein said processing means provides said output pulses by directing said first and second electrical pulses to said converting means whenever the total number of said first and second electrical pulses produced during said selected time interval exceeds the total number of said third electrical pulses produced during said selected time interval.

5. The apparatus of claim 3 further including: totalizing means operatively coupled to said processing means and adapted to respond to said output pulses for providing a signal representative of the actual depth of such a drill bit in a borehole during said selected time interval.

6. The apparatus of claim 3 wherein said processing means include: reversible pulse-counting means having an addition input, a subtraction input, and an output and operatively arranged for alternatively providing a first gate-control signal at said output whenever pulses are received by one of said inputs and providing a second gate-control signal at said output so long as the number of pulses received by said one input exceeds the number of pulses received by the other of said inputs, means adapted for coupling said third electrical pulses to said one pulse-counting means input, and gating means adapted to receive said first and second electrical pulses and having a first output for directing said first and second electrical pulses as said output pulses to said converting means in response to said first gatecontrol signal and a second output for directing said first and second electrical pulses to said other pulsecounting means input in response to said second gatecontrol signal.

7. The apparatus of claim 3 wherein said processing means include: reversible pulse-counting means having an addition input, a subtraction input, and an output and operatively arranged for alternatively providing a first gate-control signal at said output for a predetermined state of said pulse-counting means and providing a second gate-control signal at said output for another predetermined state of said pulse-counting means,

tion-responsive means meansadapted to be coupled to a drill string and remeansadapted for coupling said third electrical pulses tonne of said inputs of said pulse-counting means, and gating means adapted to receive said first and second electrical pulses and having a first output for directing said first and second electrical pulses as said output pulses to said converting means in response to said first gate-control signal and a second output for directing said first and second electrical pulses to the other of said inputs of said pulse-counting means in response to said second gate-control signal.

8.The apparatus of claim 3 wherein said converting means include circuit means adapted for determining the frequency of said output pulses during said selected time interval to provide said output signal.

9. The apparatus of claim 3 wherein said displacement-responsive means are further adapted for alternatively producing a series of fourth electrical pulses respectively corresponding to an incremental decrease in the overalllength of a drill string in a borehole during a drilling operation; and said processing means are adapted for providing said output pulses so long as the total number of said first and second electrical pulses produced during said selected time interval exceeds the totalnumber of said third and fourth electrical pulses produced during said selected time interval.

10. The apparatus of claim 3 wherein said elongainclude: tension-responsive sponsive to tension variations therein for producing electrical signals representative of increases and decreases of the overall length of such a drill string; and signal-differentiating means coupled to said tensionresponsive means and adapted to receive said electrical signals as well as first and second comparison signals of selected equal magnitude for alternatively producing said second electrical pulses only so long as the magnitude of said electrical signals is greater than the magnitude of said first comparison signal and producing said third electrical pulses only so long as the magnitude of said electrical signals is less than the magnitude of said second comparison signal.

11. The apparatus of claim wherein said electrical signals are analog voltages and said signaldifferentiating means include: first signal-comparison means having a first input coupled to said tensionresponsive means for receiving said electrical signals therefrom, a second input adapted for receiving feedback signals for comparison with said electrical signals,

and an output adapted for providing output signals representative of the differences between signals applied to said first and second inputs; second signalcomparison means having a first signal input coupled to said output of said first signal-comparison means for receiving said output signals therefrom, second and third signal inputs respectively adapted for receiving said first and second comparison signals, a first signal output adapted for providing a first output signal so long as signals applied to said first signal input exceed said first comparison signal, and a second signal output adapted for providing a second output signal so long as signals applied to said first signal input are less than said second comparison signal; pulse-generating means coupled to said first and second signal outputs and responsive to said first and second output signals for alternatively producing said second and third electrical pulses; and feedback means coupled between said pulse-generating means and said second input of said first signal-comparison means and adapted for regulating said first signal-comparison means to discontinue the production of said second and third electrical pulses whenever either the total number of either said second pulses or the total number of said third pulses produced during said selected time interval equal the total elongation or contraction during said selected time interval of a drill string coupled to said tensionresponsive means.

12. The apparatus of claim 10 wherein said electrical signals are analog voltages and said signaldifferentiating means include: first signal-comparison means having a f rst input coupled to said tensionresponsive means for receiving said electricalsignals therefrom, a second input adapted for receiving feedback signals for comparison with said electrical signals, and an output adapted for providing output signals representative of the differences between signals applied to said first and second inputs; second signalcomparison means having a first signal input coupled to said output of said first signal-comparison means for receiving said output signals therefrom, second and third signal inputs respectively adapted for receiving said first and second comparison signals, a first signal output adapted for providing a first output signal so long as signals applied to said first signal input exceed said first comparison signal, and a second signal output adapted for providing a second output signal so long as signals applied to said first signal input are less than said second comparison signal; pulse-generating means coupled to said first and second signal outputs and responsive to said first and second output signals for al ternatively producing said second and third electrical pulses; and feedback means coupled between said pulse-generating means and said second input of said first signal-comparison means and adapted for regulating said first signal-comparison means by producing second analog voltages equal to said first-mentioned analog voltages to discontinue the production of said second and third electrical pulses whenever the total number of said second pulses less the total number of said third pulses produced during said selected time interval represents said analog voltage.

13. A method for determining at least one function representative of the penetration into earth formations by a drill bit suspended from a drill string in a borehole penetrating said earth formations and comprising the steps of: successively determining the incremental lengths of said drill string moved into said borehole from the surface while said drill bit is drilling into said earth formations for deriving a first signal proportional to successive increases in the overall length of said drill string in said borehole; successively determining the incremental changes in the overall length of said drill string due to variations in tensional forces acting thereon for alternatively deriving either a second signal proportional to successive elongational increases in the overall length of said drill string or a third signal proportional to successive contractional decreases in the overall length of said drill string; and alternatively combining said first and second signals with one another for deriving an output signal representative of overall increases in the length of said drill string upon further advancement of said drill bit into said earth formations and combining said first and second signals with said third signal upon each occurrence of said third signal for selectively discontinuing further transmission of said output signal and then algebraically summing said combined signals only so long as the summation of further length increases respectively represented by subsequently-occurring first and second signals is less than the summation of further length decreases represented by subsequently-occurring third signals.

14. The method of claim 13 wherein said incremental length changes are determined by measuring incremental changes in tensional forces imposed on said drill string, and multiplying said incremental force changes by a co-efficient representative of the rate of elongation and said overall length of said drill string.

15. The method of claim 13 further including the step of: averaging said output signal over a selected time interval for determining the rate of penetration of said drill bit during said selected time interval.

16. The method of claim 13 further including the step of: totalizing said output signal over a selected time interval for determining the total downward advancement of said drill bit during said selected time interval.

17. A method for determining the rate of penetration into earth formations by a drill bit suspended from a drill string in a borehole penetrating said earth formations and comprising the steps of: successively determining the incremental lengths of said drill string moved into and out of said borehole from the surface during a selected time interval while said drill bit is drilling into said earth formations for alternatively deriving either a first signal proportional to successive increases in the overall length of said drill string in said borehole during said selected time interval or a second signal proportional to successive decreases in the overall length in said borehole during said selected time interval; successively determining the incremental changes in the overall length of said drill string due to variations in tensional forces acting thereon during said selected time interval for alternatively deriving either a third signal proportional to successive elongational increases in the overall length of said drill string during said selected time interval or a fourth signal proportional to successive contractional decreases in the overall length of said drill string during said selected time interval; alternatively combining said first and third signals with one another for deriving an output signal representative of overall increases in the length of said drill string upon further advancement of said drill bit into said earth formations during said selected time interval and combining said first and third signals with said second and fourth signals upon each occurrence of either of said second and fourth signals for selectively discontinuing transmission of said output signal and then algebraically summing said combined signals only so long as the summation of further length increases respectively represented by subsequently-occurring first and third signals is less than the summation of further length decreases represented by subsequentlyoccurring second and fourth signals; and averaging said output signal over said selected time interval for determining the rate of penetration of said drill bit during said selected time interval.

18. The method of claim 17 further including the step of: totalizing said output signal over said selected time interval for determining the total downward advancement of said drill bit during said selected time interval.

19. A method for determining the rate of advancement into earth formations by a drill bit suspended from a drill string in a borehole penetrating said earth formations and comprising the steps of: successively measuring the incremental displacements of the surface end of said drill string in relation to said borehole for alternatively producing either a series of first electrical pulses respectively corresponding to an incremental increase to the length of said drill string in said borehole or a series of second electrical pulses respectively corresponding to an incremental decrease in the length of said drill string; successively measuring the incremental changes in tensional forces acting on said drill string while said drill bit is drilling into said earth formations and converting said force changes to corresponding increments of length for alternatively producing either a series of third electrical pulses respectively corresponding to an incremental elongation of said drill string or a series of fourth electrical pulses respectively corresponding to an incremental contraction of said drill string; comparing the total of said first and third electrical pulses occurring during a time interval following the occurrence of one of said second or fourth electrical pulses with the total of said second and fourth electrical pulses occurring during said time interval; once the total of said first and third'electrical pulses counted during said time interval equal the total of said second and fourth electrical pulses counted during said time interval, producing a series of output pulses respectively corresponding to a net incremental increase in the overall length of said drill string until another one of said second or fourth electrical pulses occur; and successively converting said output pulses to a signal representative of the rate of downward advancement of said drill bit into said earth formations during a selected time interval.

20. The method of claim 19 further including the step of: totalizing said output pulses for providing an indication of the actual depth of said drill bit in said borehole.

21. Apparatus adapted for determining the rate of penetration into earth formations by a drill bit suspendedfrom a drill string in a borehole penetrating such formations and comprising: displacementresponsive means adapted for producing first signals representative of a downward movement of an incremental length of a drill string into a borehole during a drilling operation; elongation-responsive means adapted for alternatively producing either second signals representative of incremental elongations of the overall length of a drill string in a borehole during a drilling operation or third signals representative of incremental contractions of the overall length of a drill string in a borehole during a drilling operation; processing means operatively coupled to said displacementresponsive means and to said elongation-responsive means and adapted to respond to said signals produced thereby for alternatively combining said first and second signals for providing output signals representative of incremental increases in the overall length of a drill string in a borehole during a drilling operation in the absence of said third signals during a selected time interval or combining said first and second signals with said third signals upon each occurrence thereof during said selected time interval for discontinuing further transmission of said output signals until the algebraic summation of further incremental length increases represented by subsequently-occurring first and second signals equals further incremental length decreases represented by subsequently-occurring third signals; and converting means operatively coupled to said protalizing means operatively coupled to said processing means and adapted to respond to said output signals for providing signals representative of the actual depth of such a drill bit in a borehole during said selected time interval. 

1. A method for determining the rate of advancement into earth formations by a drill bit suspended from a drill string in a borehole penetrating said earth formations and comprising the steps of: successively measuring the downward incremental displacments of the surface end of said drill string being moved into said borehole while said drill bit is drilling into said earth formations for producing a series of first electrical pulses respectively corresponding to an incremental increase to the length of said drill string in said borehole; successively measuring the incremental changes in tensional forces acting on said drill string while said drill bit is drilling into said earth formations and converting said force changes to correspondiNg increments of length of alternatively producing either a series of second electrical pulses respectively corresponding to an incremental elongation of said drill string or a series of third electrical pulses respectively corresponding to an incremental contraction of said drill string; comparing the total of said first and second electrical pulses occurring during a time interval after the occurrence of one of said third electrical pulses with the total of said third electrical pulses occurring during said time interval; once the total of said first and second electrical pulses counted during said time interval equal the total of said third electrical pulses counted during said time interval, successively processing said first and second electrical pulses for producing a series of electrical output pulses respectively corresponding to a net incremental increase in the overall length of said drill string until the occurrence of another one of said third electrical pulses; and successively converting said output pulses to a signal representative of the rate of downward advancement of said drill bit into said earth formations during a selected time interval.
 2. The method of claim 1 further including the step of: totalizing said output pulses for providing an indication of the actual depth of said drill bit in said borehole.
 3. Apparatus adapted for determining the rate of penetration into earth formations by a drill bit suspended from a drill string in a borehole penetrating such formations and comprising: displacement-responsive means adapted for producing a series of first electrical pulses respectively corresponding to a downward movement of an incremental length of a drill string into a borehole during a drilling operation; elongation-responsive means adapted for alternatively producing either a series of sound electrical pulses respectively corresponding to an incremental elongation of the overall length of a drill string in a borehole during a drilling operation or a series of third electrical pulses respectively corresponding to an incremental contraction of the overall length of a drill string in a borehole during a drilling operation; processing means operatively coupled to said displacement-responsive means and to said elongation-responsive means and adapted to respond to said electrical pulses produced thereby for providing a series of electrical output pulses respectively corresponding to a net incremental increase in the overall length of a drill string in a borehole during a drilling operation so long as the total number of said first and second electrical pulses produced during a selected time interval exceeds the total number of said third electrical pulses produced during said selected time interval; and converting means operatively coupled to said processing means and adapted to respond to said output pulses for providing an output signal representative of the rate of penetration into earth formations by a drill bit while drilling a borehole during said selected time interval.
 4. The apparatus of claim 3 wherein said processing means provides said output pulses by directing said first and second electrical pulses to said converting means whenever the total number of said first and second electrical pulses produced during said selected time interval exceeds the total number of said third electrical pulses produced during said selected time interval.
 5. The apparatus of claim 3 further including: totalizing means operatively coupled to said processing means and adapted to respond to said output pulses for providing a signal representative of the actual depth of such a drill bit in a borehole during said selected time interval.
 6. The apparatus of claim 3 wherein said processing means include: reversible pulse-counting means having an addition input, a subtraction input, and an output and operatively arranged for alternatively providing a first gate-control signal at said output whenever pulses are received by one of said inputs and providing A second gate-control signal at said output so long as the number of pulses received by said one input exceeds the number of pulses received by the other of said inputs, means adapted for coupling said third electrical pulses to said one pulse-counting means input, and gating means adapted to receive said first and second electrical pulses and having a first output for directing said first and second electrical pulses as said output pulses to said converting means in response to said first gate-control signal and a second output for directing said first and second electrical pulses to said other pulse-counting means input in response to said second gate-control signal.
 7. The apparatus of claim 3 wherein said processing means include: reversible pulse-counting means having an addition input, a subtraction input, and an output and operatively arranged for alternatively providing a first gate-control signal at said output for a predetermined state of said pulse-counting means and providing a second gate-control signal at said output for another predetermined state of said pulse-counting means, means adapted for coupling said third electrical pulses to one of said inputs of said pulse-counting means, and gating means adapted to receive said first and second electrical pulses and having a first output for directing said first and second electrical pulses as said output pulses to said converting means in response to said first gate-control signal and a second output for directing said first and second electrical pulses to the other of said inputs of said pulse-counting means in response to said second gate-control signal.
 8. The apparatus of claim 3 wherein said converting means include circuit means adapted for determining the frequency of said output pulses during said selected time interval to provide said output signal.
 9. The apparatus of claim 3 wherein said displacement-responsive means are further adapted for alternatively producing a series of fourth electrical pulses respectively corresponding to an incremental decrease in the overall length of a drill string in a borehole during a drilling operation; and said processing means are adapted for providing said output pulses so long as the total number of said first and second electrical pulses produced during said selected time interval exceeds the total number of said third and fourth electrical pulses produced during said selected time interval.
 10. The apparatus of claim 3 wherein said elongation-responsive means include: tension-responsive means adapted to be coupled to a drill string and responsive to tension variations therein for producing electrical signals representative of increases and decreases of the overall length of such a drill string; and signal-differentiating means coupled to said tension-responsive means and adapted to receive said electrical signals as well as first and second comparison signals of selected equal magnitude for alternatively producing said second electrical pulses only so long as the magnitude of said electrical signals is greater than the magnitude of said first comparison signal and producing said third electrical pulses only so long as the magnitude of said electrical signals is less than the magnitude of said second comparison signal.
 11. The apparatus of claim 10 wherein said electrical signals are analog voltages and said signal-differentiating means include: first signal-comparison means having a first input coupled to said tension-responsive means for receiving said electrical signals therefrom, a second input adapted for receiving feedback signals for comparison with said electrical signals, and an output adapted for providing output signals representative of the differences between signals applied to said first and second inputs; second signal-comparison means having a first signal input coupled to said output of said first signal-comparison means for receiving said output signals therefrom, second and third signal inputs respectively adapted for receiving said First and second comparison signals, a first signal output adapted for providing a first output signal so long as signals applied to said first signal input exceed said first comparison signal, and a second signal output adapted for providing a second output signal so long as signals applied to said first signal input are less than said second comparison signal; pulse-generating means coupled to said first and second signal outputs and responsive to said first and second output signals for alternatively producing said second and third electrical pulses; and feedback means coupled between said pulse-generating means and said second input of said first signal-comparison means and adapted for regulating said first signal-comparison means to discontinue the production of said second and third electrical pulses whenever either the total number of either said second pulses or the total number of said third pulses produced during said selected time interval equal the total elongation or contraction during said selected time interval of a drill string coupled to said tension-responsive means.
 12. The apparatus of claim 10 wherein said electrical signals are analog voltages and said signal-differentiating means include: first signal-comparison means having a first input coupled to said tension-responsive means for receiving said electrical signals therefrom, a second input adapted for receiving feedback signals for comparison with said electrical signals, and an output adapted for providing output signals representative of the differences between signals applied to said first and second inputs; second signal-comparison means having a first signal input coupled to said output of said first signal-comparison means for receiving said output signals therefrom, second and third signal inputs respectively adapted for receiving said first and second comparison signals, a first signal output adapted for providing a first output signal so long as signals applied to said first signal input exceed said first comparison signal, and a second signal output adapted for providing a second output signal so long as signals applied to said first signal input are less than said second comparison signal; pulse-generating means coupled to said first and second signal outputs and responsive to said first and second output signals for alternatively producing said second and third electrical pulses; and feedback means coupled between said pulse-generating means and said second input of said first signal-comparison means and adapted for regulating said first signal-comparison means by producing second analog voltages equal to said first-mentioned analog voltages to discontinue the production of said second and third electrical pulses whenever the total number of said second pulses less the total number of said third pulses produced during said selected time interval represents said analog voltage.
 13. A method for determining at least one function representative of the penetration into earth formations by a drill bit suspended from a drill string in a borehole penetrating said earth formations and comprising the steps of: successively determining the incremental lengths of said drill string moved into said borehole from the surface while said drill bit is drilling into said earth formations for deriving a first signal proportional to successive increases in the overall length of said drill string in said borehole; successively determining the incremental changes in the overall length of said drill string due to variations in tensional forces acting thereon for alternatively deriving either a second signal proportional to successive elongational increases in the overall length of said drill string or a third signal proportional to successive contractional decreases in the overall length of said drill string; and alternatively combining said first and second signals with one another for deriving an output signal representative of overall increases in the length of said drill string upon further advancement of said drilL bit into said earth formations and combining said first and second signals with said third signal upon each occurrence of said third signal for selectively discontinuing further transmission of said output signal and then algebraically summing said combined signals only so long as the summation of further length increases respectively represented by subsequently-occurring first and second signals is less than the summation of further length decreases represented by subsequently-occurring third signals.
 14. The method of claim 13 wherein said incremental length changes are determined by measuring incremental changes in tensional forces imposed on said drill string, and multiplying said incremental force changes by a co-efficient representative of the rate of elongation and said overall length of said drill string.
 15. The method of claim 13 further including the step of: averaging said output signal over a selected time interval for determining the rate of penetration of said drill bit during said selected time interval.
 16. The method of claim 13 further including the step of: totalizing said output signal over a selected time interval for determining the total downward advancement of said drill bit during said selected time interval.
 17. A method for determining the rate of penetration into earth formations by a drill bit suspended from a drill string in a borehole penetrating said earth formations and comprising the steps of: successively determining the incremental lengths of said drill string moved into and out of said borehole from the surface during a selected time interval while said drill bit is drilling into said earth formations for alternatively deriving either a first signal proportional to successive increases in the overall length of said drill string in said borehole during said selected time interval or a second signal proportional to successive decreases in the overall length in said borehole during said selected time interval; successively determining the incremental changes in the overall length of said drill string due to variations in tensional forces acting thereon during said selected time interval for alternatively deriving either a third signal proportional to successive elongational increases in the overall length of said drill string during said selected time interval or a fourth signal proportional to successive contractional decreases in the overall length of said drill string during said selected time interval; alternatively combining said first and third signals with one another for deriving an output signal representative of overall increases in the length of said drill string upon further advancement of said drill bit into said earth formations during said selected time interval and combining said first and third signals with said second and fourth signals upon each occurrence of either of said second and fourth signals for selectively discontinuing transmission of said output signal and then algebraically summing said combined signals only so long as the summation of further length increases respectively represented by subsequently-occurring first and third signals is less than the summation of further length decreases represented by subsequently-occurring second and fourth signals; and averaging said output signal over said selected time interval for determining the rate of penetration of said drill bit during said selected time interval.
 18. The method of claim 17 further including the step of: totalizing said output signal over said selected time interval for determining the total downward advancement of said drill bit during said selected time interval.
 19. A method for determining the rate of advancement into earth formations by a drill bit suspended from a drill string in a borehole penetrating said earth formations and comprising the steps of: successively measuring the incremental displacements of the surface end of said drill string in relation to said borehole for alternatively producing either a series of first electrical pulses reSpectively corresponding to an incremental increase to the length of said drill string in said borehole or a series of second electrical pulses respectively corresponding to an incremental decrease in the length of said drill string; successively measuring the incremental changes in tensional forces acting on said drill string while said drill bit is drilling into said earth formations and converting said force changes to corresponding increments of length for alternatively producing either a series of third electrical pulses respectively corresponding to an incremental elongation of said drill string or a series of fourth electrical pulses respectively corresponding to an incremental contraction of said drill string; comparing the total of said first and third electrical pulses occurring during a time interval following the occurrence of one of said second or fourth electrical pulses with the total of said second and fourth electrical pulses occurring during said time interval; once the total of said first and third electrical pulses counted during said time interval equal the total of said second and fourth electrical pulses counted during said time interval, producing a series of output pulses respectively corresponding to a net incremental increase in the overall length of said drill string until another one of said second or fourth electrical pulses occur; and successively converting said output pulses to a signal representative of the rate of downward advancement of said drill bit into said earth formations during a selected time interval.
 20. The method of claim 19 further including the step of: totalizing said output pulses for providing an indication of the actual depth of said drill bit in said borehole.
 21. Apparatus adapted for determining the rate of penetration into earth formations by a drill bit suspended from a drill string in a borehole penetrating such formations and comprising: displacement-responsive means adapted for producing first signals representative of a downward movement of an incremental length of a drill string into a borehole during a drilling operation; elongation-responsive means adapted for alternatively producing either second signals representative of incremental elongations of the overall length of a drill string in a borehole during a drilling operation or third signals representative of incremental contractions of the overall length of a drill string in a borehole during a drilling operation; processing means operatively coupled to said displacement-responsive means and to said elongation-responsive means and adapted to respond to said signals produced thereby for alternatively combining said first and second signals for providing output signals representative of incremental increases in the overall length of a drill string in a borehole during a drilling operation in the absence of said third signals during a selected time interval or combining said first and second signals with said third signals upon each occurrence thereof during said selected time interval for discontinuing further transmission of said output signals until the algebraic summation of further incremental length increases represented by subsequently-occurring first and second signals equals further incremental length decreases represented by subsequently-occurring third signals; and converting means operatively coupled to said processing means and adapted to respond to said output signals for providing at least one indication representative of the rate of penetration into earth formations by a drill bit while drilling a borehole during said selected time interval.
 22. The apparatus of claim 21 further including: totalizing means operatively coupled to said processing means and adapted to respond to said output signals for providing signals representative of the actual depth of such a drill bit in a borehole during said selected time interval. 